An array of low-background 3He proportional counters for the Sudbury Neutrino Observatory

An array of Neutral-CurrentDetectors (NCDs) has been built in order to make a unique measurement of the total active flux of solar neutrinosin the Sudbury Neutrino Observatory(SNO). Data in the third phaseof the SNO experiment were collected between November 2004 and 2006, after the NCD array was added to improve the neutral-currentsensitivity of the SNO detector. This array consisted of 36 strings of proportional countersfilled with a mixture of He-3 and CF4 gas capable of detecting the neutrons liberated by the neutrino-deuteron neutral-currentreaction in the D2O, and four strings filled with a mixture of He-4 and CF4 gas for background measurements. The proportional counterdiameter is 5 cm. The total deployed array length was 398 m. The SNO NCD array is the lowest-radioactivity large array of proportional countersever produced. This article describes the design, construction, deployment, and characterization of the NCD array, discusses the electronics and data acquisition system, and considers event signatures and backgrounds.